Context: Innovative ZLD technologies can power wastewater recovery through solar thermal application.
Issue

• As a rapidly developing country, India is struggling with the growing challenge of managing its hazardous waste. 
• In 2019, the Central Pollution Control Board (CPCB) released the National Inventory Report of industries producing hazardous waste.
• The report states that there are 56 thousand industrial units generating hazardous waste in India, producing a total of 7.17 million tonnes of hazardous waste annually (during 2016–17). 

Possible Reason

• Majority of the production processes in industries happen in a liquid medium, implying that the waste is primarily in the form of liquid effluent. 

About Zero Liquid Discharge (ZLD)

• As per the waste management hierarchy, effluent is supposed to be treated, and water and minerals recovered and reused. 
• The end product — which cannot be recovered — is treated and made chemically inert before being safely disposed in a secured landfill. 
• In common parlance, this process of recycling and reuse of waste in water-intensive industries is known as Zero Liquid Discharge (ZLD).
• So far, its use has been limited to units of major multi-national companies like Unilever and Procter & Gamble.

Challenge to adoption of ZLD

• One of the major impediments in wider adoption of ZLD is its operating cost in terms of fuel expenses and one-time high capital cost (during installation).
• ZLD mainly incorporates effluent treatment, reverse osmosis (RO) and multi-effect evaporators (MEE). 
• Among the three systems that combine to form a ZLD, MEEs are the most expensive to operate and are the greatest challenge in the water treatment industry.
• It is also hampered by the fact that they emit significant quantities of carbon dioxide and are powered by fossil fuels.  

Solution to this challenge

• Membrane distillation (MD) is an advanced form of membrane-assisted MEE.
• An MD system is powered by hot water instead of steam, so it can be powered by commonly used solar thermal collectors, thereby reducing the need of boilers (that will only be used during monsoon, when the solar option is limited). 
• MD technology comprises a multistage process (generally five stages). 
• The basic operating principle of MD is that by reducing pressure in a closed chamber, the boiling point of water is reduced.
• As per Ministry, MEE systems are currently operational such as pharmaceuticals and metal finishing.

Road Ahead

• The water and energy crisis our world is facing are intertwined. They need innovative synergetic solutions and new business models such as “water as a service” wherein a company installs the system and charges clients on the basis of the amount of water that has been treated.